Flexible or “flex” circuits are used in a wide variety of applications where an electrical circuit must bend around corners or be flexed during operation. Flex circuits are thin, light weight, flexible and exhibit high routability. Traditionally, polyimide films have been used as substrates in the manufacture of flex circuits due to their good thermal stability and mechanical strength. Other properties of polyimide films, however, limit the speed or frequency at which electric components mounted thereto can operate.
Liquid crystal polymer (“LCP”) has been developed in recent years as a replacement for polyimide films in flex circuits. LCP is a thermoplastic aromatic polyester which is thermally stable, with an upper use temperature in excess of 250° C. and good inherent flame retardant properties. LCP films, in comparison to polyimide films, have about one-tenth of the moisture uptake and a lower coefficient of humidity expansion. Lower moisture absorption leads to higher frequency signal and data processing stability. Additionally, LCP films have a lower dielectric constant and a lower loss or dissipation factor over the functional frequency range of 1 kHz to 45 GHz, with negligible moisture effects, compared to polyimide films.
The fabrication of flex circuits with LCP films is expected to lead to their use in more demanding environments where moisture and other contaminants are prevalent. Particularly in such types of applications, the circuit elements applied to the LCP substrate of the flex circuit must be protected from damage. Soldermask coatings, which have been employed to provide protection from moisture and contaminants in polyimide films, have been considered for use with LCP substrates. Additionally, due to the thermoplastic nature of LCP, the application of an LCP film cover layer to an LCP substrate has been proposed as a means of encapsulating circuit elements. With respect to LCP cover layers, current practice is to employ an air knife or laser to create localized heating of the LCP cover layer and LCP substrate along the periphery of the flex circuit. A number of problems can arise from this approach. A failure of the seal between the cover layer and substrate at any point along the periphery of the flex circuit can expose all of the circuit elements to moisture, chemicals or other contaminants. If the air between the cover layer and substrate is not fully removed, pressurization of the flex circuit which would occur in underwater applications, for example, could compress such air and create a bubble potentially resulting in a rupture of the cover layer and/or substrate thus creating a failure of the entire circuit.
The melt temperature of LCP material is approximately 283° C., and soldermask coatings are also applied at relatively high temperatures. While a number of standard circuit elements are not affected by high temperatures, components such as micro-electrical-mechanical-system (“MEMS”) sensors, infra-red sensors and a variety of other circuit elements are temperature sensitive and can be damaged or destroyed upon exposure to elevated temperatures. There is a need for an efficient and dependable method and apparatus capable of individually sealing or encapsulating the electrical components of circuits which employ an LCP substrate, while protecting heat sensitive components of the circuit from damage due to the temperatures at which the sealing process is performed.